Deep Search for Molecular Oxygen in TW Hya

Abstract

The dominant form of oxygen in cold molecular clouds is gas-phase carbon monoxide (CO) and ice-phase water (H _2 O). Yet, in planet-forming disks around young stars, gas-phase CO and H _2 O are less abundant relative to their interstellar medium values, and no other major oxygen-carrying molecules have been detected. Some astrochemical models predict that gas-phase molecular oxygen (O _2 ) should be a major carrier of volatile oxygen in disks. We report a deep search for emission from the isotopologue ^16 O ^18 O ( N _J = 2 _1 − 0 _1 line at 233.946 GHz) in the nearby protoplanetary disk around TW Hya. We used imaging techniques and matched filtering to search for weak emission but do not detect ^16 O ^18 O. Based on our results, we calculate upper limits on the gas-phase O _2 abundance in TW Hya of (6.4–70) × 10 ^−7 relative to H, which is 2–3 orders of magnitude below solar oxygen abundance. We conclude that gas-phase O _2 is not a major oxygen carrier in TW Hya. Two other potential oxygen-carrying molecules, SO and SO _2 , were covered in our observations, which we also do not detect. Additionally, we report a serendipitous detection of the C ^15 N N _J = 2 _5/2 − 1 _3/2 hyperfine transitions, F = 3 − 2 and F = 2 − 1, at 219.9 GHz, which we found via matched filtering and confirm through imaging.

Keywords

• Astrochemistry
• Protoplanetary disks